1. Field of the Invention
The present invention relates to a printing ink containing a first coloring material and a second coloring material, in which at least one of the coloring materials includes a fluorescent coloring material and the first coloring material and the second coloring material form a separated state on a recording medium, and a printing ink capable of providing a printed image with improved fluorescent properties. The present invention also relates to a recorded image and an image forming method utilizing such ink. More specifically, the present invention relates to related technologies based on a novel technical idea for solving decrease in fluorescence and for enhancing fluorescence properties by separating the fluorescent region and the coloring region, more specifically by forming a region on a recording medium where dots of coagulated first coloring material are scattered in a region of the fixed second coloring material thereby significantly reducing absorption of the fluorescent emission energy from the fluorescent coloring material by the coexistent coloring material. It also relates to interrelated technologies based on a novel concept for improving fluorescence emission characteristics of a second fluorescent coloring material employed in a printing ink including a first fluorescent coloring material that, on excitation at a predetermined excitation wavelength, fluoresces in a wavelength range including a predetermined wavelength utilized for measurement or determination.
2. Related Background Art
Conventionally, water-soluble coloring materials have been used as the coloring material for aqueous inks, so that most inks show excellent color development and satisfactory resistance to coagulation or association of the coloring material, as well as good reliability such as clogging resistance and stability with time. On the other hand, these inks are insufficient in fastness of a recorded image, such as water resistance. Regarding fluorescent properties, in particular, the good solubility of the coloring material allows satisfactory fluorescence, but the intensity cannot be enhanced because of the concentration quenching (a phenomenon of fluorescence decrease when the content of the coloring material exceeds a certain amount). For example, U.S. Pat. No. 6,176,908 (patent reference 1) proposes an ink utilizing a polymer dispersant, a pigment and a fluorescent dye.
In the above proposal; however, when an ink droplet is applied onto a recording medium, the polymer dispersant, the pigment and the fluorescent dye are deposited in a solid coagulation state (FIG. 2) in the entire system, whereby the image density of a record increases but the color-developing property including fluorescence emission is not so much improved as expected because of the aforementioned concentration quenching phenomenon.
Such a drawback might be overcome, if limited to improvement of PMU (an index representing the fluorescence intensity) described in the above proposal, by controlling the print area to be measured for the fluorescent intensity, but it will be difficult in the ordinary use.
Meanwhile, for improving the water resistance, it has been proposed to employ a coloring material having carboxyl group as a water-soluble group. This proposal utilizes dissociation characteristics of the carboxylic acid constituting the water-soluble group, and, when pH of the ink shifts acidic on the recording medium, the solubility of the coloring material in water decreases and the coloring material coagulates to improve the water resistance.
In such a proposal, however, the water resistance is improved by coagulation or association of the coloring material due to decreased solubility of the coloring material in water, so there arises a problem that the color-developing properties of the coloring material is lowered. In particular, it is difficult to improve the fluorescent emission because of coagulation or association of the coloring material.
Also in order to avoid the aforementioned concentration quenching (a phenomenon of decrease in fluorescence when the content of the coloring material exceeds a certain amount) and to increase the image density of a record, for example, WO 02/092707 (patent reference 2) proposes a method of combined use of plural coloring materials in such a manner that the fluorescence emission wavelength range of a coloring material overlaps as less as possible with the absorption wavelength range of a coexistent coloring material to improve fluorescent emission of a recorded image.
Such proposals can increase the fluorescent intensity and the density of the recorded image to a certain extent but still insufficient. Also in the WO 02/092707 (patent reference 2), the fastness of the recorded image is still insufficient requiring much improvement in the water resistance and the light fastness.
Recently, in order to improve fastness of the recorded image such as water resistance and light fastness, there has been proposed an aqueous ink employing a combination of a water-dispersible color material such as carbon black or an organic pigment and a colored resin incorporating a water-soluble coloring material. For example, Japanese Unexamined Patent Publication No. 8-239610 (patent reference 3) discloses an aqueous ink containing a water-dispersible coloring material dispersed in an aqueous system by the action of a resin and an emulsion of a water-soluble coloring material emulsified with an emulsifier or a resin for enhancing color-developing property. Also WO 02/092707 (patent reference 2) proposes utilization of a pigment including a fluorescent coloring material.
When such an ink is placed on a recording medium, water or the liquid medium evaporates from or penetrates and diffuses into the recording medium to cause coagulation of the dispersed coloring material, thereby the fastness of the recorded image such as the water resistance or light fastness is improved. At the same time, the water-soluble coloring material incorporated in a resin etc. will not readily coagulate or associate by itself, thereby providing a satisfactory color-developing property, particularly fluorescence emission. However, since the ink contains a large amount of a resin, emulsifier etc., the ink-viscosity tends to increase, and, such coloring materials-employed in a larger amount tends to deteriorate the fixing-drying properties of the recorded image.
Also such ink is deficient in the resistance to solidification and tends to show a solidification of the pigment component when left standing in an environment of a normal temperature. For example, when such ink is used in an ink jet head, destruction of the stable dispersion of the coloring material tends to occur by evaporation of the water or liquid medium of the ink in the vicinity of the nozzle of the head to cause coagulation of the coloring material, whereby ink solidification or a nozzle clogging may occur. Also in an ink jet recording head for discharging an ink droplet by the action of thermal energy applied to the ink, the dispersed coloring material tends to be deposited on a heater, thereby the reliability of the ink discharge property of the head is significantly deteriorated.
Thus, the above-mentioned proposals can improve the water resistance and the light fastness for the water-soluble dye in the record, but may deteriorate the fixing-drying property of the recorded image and the resistance to solidification when the recording density of the image is raised. Also when such ink is used for the ink jet recording method, nozzle clogging or discharge failure may occur.
In recent years, various applications have been requested for ink. As such applications, in addition to formation of beautiful color images, there are proposed, for example, use of fluorescence ink for providing information (such as security information) in addition to visual information, by printing information such as characters, numbers, symbols, or bar-codes with such an ink on a recording medium and irradiating UV light of an appropriate wavelength to generate colored fluorescence from the fluorescence ink. Specifically, in a system for reading out authentication (anti-counterfeit) information or security information using-an apparatus to excite fluorescence and read the emission intensity thereof, a fluorescent coloring agent is excited by excitation light of a predetermined wavelength (e.g., 254 nm) to fluoresce, and the fluorescence is determined or measured.
Regarding the coloring materials in the ink, dyes can provide a predetermined color easily but occasionally poor water-resistance, while pigments can give excellent water resistance but not the predetermined color tone occasionally. In view of the above, there is proposed ink containing both dye and pigment to obtain an ink capable of providing an image excellent in both water-resistance and color tone. For example, Japanese Examined. Patent Publication S60-45669 (Patent reference 4) discloses a recording liquid that contains a water-soluble red dye (e.g., Acid Red 52) and a red pigment as recording agents, and a polymer dispersant for dispersing the pigment in a liquid medium.
In the mailing systems of the United States, printing with fluorescent red is common, and a dye such as Acid Red 52 (AR52), which is described in the above publication, is used as a fluorescent dye. U.S. Pat. No. 6,176,908 (Patent reference 1) discloses an ink containing a fluorescent dye, a pigment and a polymer as a dispersant for the pigment, exemplifying AR52 as a fluorescent dye. It should be noted, it had been a well-known design matter long before U.S. Pat. No. 6,176,908 to adjust the final color shade according to human visual sensation by combination of dyes.
U.S. Pat. No. 6,176,908 provides an inkjet ink containing a pigment in addition to a fluorescent dye for improving the water-resistance of the ink as with JP S60-45669B (Patent reference 4), and there are described addition of two kinds of fluorescent dyes for the known object of visible (visual) color adjustment, and also additives for improving fluorescence intensity (PMU level) to the system. Concrete ink examples of improved fluorescence intensity (PMU level) contain solvents such as water, 2-pyrrolidone, and tetraethylene glycol, and the following fluorescent coloring materials other than the pigment-related component comprised of a pigment, a polymer and tetraethylene glycol or diethylene glycol. As the fluorescent coloring materials, a combination of AR52 (0.4% by mass, 0.5% by mass to 3.0% by mass) and one of AY7, AY73, and DY96, and a combination of basic violet (RHDB) and basic yellow (BY40) are described, for example.
JP H11-80632A (Patent reference 5) discloses an invisible fluorescence aqueous ink containing three different fluorescent coloring materials (a fluorescence brightening agent, a yellow fluorescent dye of a coumarin derivative, and a red fluorescent dye of rhodamine-B or rhodamine-6G), and postcard printing using the ink. In the technical descriptions thereof, each of these three fluorescent coloring materials emits light under UV light irradiation to excite the other coloring material sequentially leading to final fluorescence emission having a wavelength peak at 587 nm. In this publication, however, there is no concrete description about excitation wavelengths, and the technical description is made using a result that the ink and the recorded image show the same fluorescence characteristics. Generally, water absorbs UV light, so that the fluorescence of a recorded image will be different from that of the ink used. Judging from this, the invention described in the publication lacks technical credibility. WO 02/092707 (Patent reference 2) discloses an ink that can form a dark image and also exhibit fluorescence of a predetermined color when exposed exciting radiation. The ink contains a plurality of dyes (e.g., red and yellow fluorescent dyes, a blue dye, and a black dye) as with the patent reference 5, but differs from the patent reference 3 in that dyes are selected such that the longer wavelength absorption band and the shorter wavelength emission band would not overlap. In this publication, the relationship between the fluorescent coloring materials is not analyzed sufficiently so that the desired fluorescence intensity cannot always be obtained. JP 2003-113331A (Patent reference 6) discloses an invention for improving the fluorescence characteristics of ink in terms of the relationship between solvents and fluorescent coloring materials. In other words, the patent reference 6 discloses a recording ink that includes two fluorescent coloring materials of the same color (there is an example where a non-fluorescent coloring material is added), two different organic solvents (e.g., glycerin and a nonionic surfactant) which have no compatibility to each other, and pure water for dissolving these components.
Patent reference 1: U.S. Pat. No. 6,176,908;
Patent-reference 2: WO 02/092707;
Patent reference 3: Japanese Unexamined Patent Publication No. 8-239610;
Patent reference 4: Japanese Examined Patent Publication No. 60-45669;
Patent reference 5: Japanese Unexamined Patent Publication No. 11-80632;
Patent reference 6: Japanese Unexamined Patent Publication No. 2003-113331.
As explained above, there has not been proposed an aqueous ink that can provide excellent water resistance, recorded image density, and particularly color-developing properties including fluorescence with high reliability. An object of the present invention is to provide an ink capable of solving the aforementioned problems.
In particular, the present invention is to provide an ink that is less affected by the absorption characteristics of a coloring material used in the ink, clarifying “the problem of fluorescence loss according to the absorption characteristics of the coloring material of joint use (light absorption due to the color of a coloring material)”, one of the major factors inhibiting the fluorescence emission, by analyzing a fixed state of the ink on the recording medium based on a novel technical concept, whereby providing a recorded image with satisfactory fluorescent emission on a recording medium and a method forming such an image.